The invention relates to a semiconductor device having a semiconductor body comprising several surface regions provided with semiconductor elements, whereby in the operating condition the surface regions have a comparatively high dissipation with respect to remaining parts of the semiconductor body.
The surface regions having a high dissipation may be, for example, parts of a power transistor, this power transistor being subdivided into a number of subtransistors in order to improve the thermal behavior of the transistor. In such power transistors, the emitter is frequently constituted by a row of finger-shaped regions, which are designated hereinafter as emitter fingers and which extend parallel to each other and in a direction at right angles to the longitudinal direction of the row in a base zone of the transistor.
As is known, in bipolar transistors with larger currents the major part of the emitter current is injected into the base through those parts of the emitter-base junction which are closest to the base contact. Parts of the emitter-base junction which are located more distant from the base contact are not or are substantially not effective due to voltage losses in the base. When the emitter is subdivided, the base contact can be given such a configuration that an emitter-base junction is obtained which has a comparatively large injecting surface area with larger currents. The base contact may then have the form, for example, of a number of base contact fingers which are interdigitated with the emitter fingers.
Power transistors of the kind described above are often provided with resistors in the emitter or base connection in order to avoid so-called "second breakdown". This effect can occur due to a local temperature increase of the emitter-base junction. At the area at which such a temperature increase occurs, even if it is small, the emitter current across the emitter-base junction will increase. This results in a local increase of the dissipation and hence in a further temperature increase. In this manner, an avalanche effect can be obtained, which results in a breakdown which may lead to the destruction of the transistor. When, for example, resistors (connected to the emitter fingers) are included in the emitter path of the transistor, in the case of any local temperature increase and the current increase connected therewith, the forward voltage across the emitter-base junction and hence the emitter current across this emitter-base junction is reduced.
In order to obtain optimum protection against second breakdown for the whole operational working range, within which it should be possible to operate the transistor, comparatively large resistors are required. However, the operating conditions are frequently such that it will be necessary to use considerably lower resistance values, as is the case when the current has to be large and at the same time the voltage drop across the resistor has to be low. In general, it can therefore be said that, when given resistance values are chosen with a view to very special operating conditions of the transistor, an optimum operation of the transistor under other operating conditions is not guaranteed.
It has been found that an important cause of "second breakdown" is the non-uniform temperature distribution obtained in the transistor during operation. It has been found, for example, that without special measures, the temperature of a transistor is lower at the periphery than at the center. As a result, "second breakdown" will generally occur at the center rather than at the periphery of the transistor. In U.S. Pat. No. 3,704,398 it has already been suggested in connection herewith to enlarge the distance between adjacent emitter fingers at the center of the transistor with respect to the distance between adjacent emitter fingers at the periphery of the transistor. By a suitable choice of the distances between the emitter fingers and hence of the thermal resistances between the various emitter fingers, a better temperature distribution in the transistor can be obtained.
Another solution, in which less space is occupied, has been suggested in Dutch Patent Application No. 770,5729, laid open for public inspection on Nov. 28, 1978, corresponding to U.S. Ser. No. 301,870. In this case, a more uniform temperature variation is obtained in a transistor by giving the emitter fingers different lengths so that, due to unequal dissipation in the emitter fingers, the uniformity in the temperature distribution across the transistor is improved in the longitudinal direction (at right angles to the emitter fingers).
As stated above, for high powers a transistor can be subdivided into a number of subtransistors. These subtransistors can then again have a structure with unequal lengths of the emitter fingers, for example, such that the length of the emitter fingers decreases from the periphery of the transistor towards the center. However, experiments have shown that especially with such high-power transistors, whereby the transistor is subdivided into a large number of subtransistors in order to obtain a sufficient current capacity at a low voltage, nevertheless thermal instabilities may occur, which lead to the "second breakdown" effect. Infrared measurements on such semiconductor devices having several dissipating subregions have also shown that the temperature variation across the semiconductor body is not optimal.
A first solution of this problem can be found in that emitter series resistors are used (or already existing emitter series resistors are enlarged). The disadvantages connected with the use of emitter series resistors have been described above.
Another solution can be found in that, when emitter fingers of different lengths are used, a more strongly pronounced difference in length (for example, even shorter fingers at the center) is chosen for those fingers. However, since for the same quantity of current the overall emitter length has to remain the same, this would only result in an even larger number of subtransistors, whereby the temperature variation across the semiconductor body would be deteriorated.